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    <title>Document</title>
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  <body>
    <script>
      // 原型对象的应用 扩展内置对象方法
      Array.prototype.sum = function () {
        var sum = 0
        for (var i = 0; i < this.length; i++) {
          sum += this[i]
        }
        return sum
      }
      // 使用此方式扩展内置对象，会覆盖原来其他方法。
      /* Array.prototype = {
        sum: function () {
          var sum = 0
          for (var i = 0; i < this.length; i++) {
            sum += this[i]
          }
          return sum
        },
      } */
      var arr = [1, 2, 3]
      console.log(arr.sum())
      console.log(Array.prototype)
      var arr1 = new Array(11, 22, 33)
      console.log(arr1.sum())
    </script>
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</html>
